Because of their relatively low cost and low density polyolefin fibers such as polypropylene are regarded as good candidate material for use in non-woven fabrics. In fact, polypropylene fibers have already found acceptance for such purpose as spun bonded, needle punched, and thermally bonded non wovens. In applications where multidirectional strength, particularly wet cross-directional strength, of the non-woven fabric is desired, however, the performance of polyolefin fiber webs leave something to be desired. This is particularly the case where the non-woven fabric is intended for use in contact with the human body, such as a diaper component or similar purpose, and where retention of good softness and absorptivity characteristics is important.
Generally speaking, cross directional strength of non-wovens depends upon
(a) the weight and number of fibers in the web, PA1 (b) the degree or amount of cross-orientation of such fibers due to manner of web formation; PA1 (c) the nature, amount, and distribution of binder used to form the fiber web, and PA1 (d) curing conditions, such as the utilization of heat.
Wet cross directional strength depends substantially upon the choice and amount of binder.
Latex-bonded non-woven fabrics are customarily composed of loosely assembled webs of synthetic fibers bound together at various points with an adhesive binder. In particular, the fiber web is obtained by carding or garnetting the fiber, followed by application of binder (usually as an aqueous solution, suspension, or dispersion) by using spray, print rolls or similar art-recognized means of application. The treated web is then dried and cured to obtain the desired non-woven fabric. This technique is customarily referred to as a "Dry Process".
Alternatively, aqueous fiber suspensions can be captured or applied onto a screen to form a wet sheet, binder being added initially to the fiber suspension and chemically precipitated onto the suspended fibers. These steps are customarily followed by using paper-making equipment such as Fourdriniers or Rotoformers, and the resulting sheet removed by vacuum transfer to a belt for drying and curing (i.e. "Wet Process").
Non-wovens formed by a Dry Process are preferred for many end products. Unfortunately, however, the Dry Process tends to minimize random orientation of fibers in favor of a general "machine" direction orientation. This results in a fabric tensile strength that is much lower in the "cross direction" then in the "Machine Direction". Cross direction tensile strength in wet tests is found to be particularly low in the absence of high concentrations of binder material. High binder concentrations, however, tend to mask or at least modify desired softness, water permeability and absorptivity characteristics of the nonwoven fabric.
A further difficulty arises from the fact that polyolefin fibers can vary considerably with respect to binder-wetting properties and also with respect to sensitivity to damage from heat curing. In this regard, polyolefin fibers such as polypropylene tend to be particularly sensitive to high curing temperatures and tend to lose valuable textural and absorption characteristics. A demand for binder concentrations in excess of about 40% by weight of fiber also acts adversely with respect to the latter properties.
It is an object of the present invention to develop a low temperature easily applicable binder system suitable for forming non woven fabrics from polyolefin-containing fiber webs.
It is a further object of this invention to increase the multidirectional strength of non-woven polyolefin-containing fabrics, particularly cross-directional strength, by utilizing an improved low temperature binder system.
Another object of the present invention is to prepare a non-woven polypropylene fabric having good cross-directional wet strength.